"The counsellors know about the abuse and we have had additional (separate) assessment appointments to address this and will start counselling individually at first with a view to couples counselling after that. Is that still not a good idea?".
No it is not a good idea at all and this is why:-
To use mediation is to subscribe to the mistaken idea that abuse is related to "misunderstandings" or lack of communication. If discussion and compromise, the mainstay of mediation, could help in any way most domestic violence situations would be long ago resolved because victims of abuse "discuss and compromise" constantly. Mediation assumes both parties will cooperate to make agreements work; the victim has always 'cooperated' with the abuser; the abuser never cooperates.
Your H will never co-operate and will use mediation as a further stick to beat you up with.
Mediation can be and is ordered by judges/courts, as can counselling and mental health evaluations. They are tools in the abuser's arsenal to be used against the victim as often as he chooses. In order for mediation to work and to not make situations worse the parties involved must have equal power and must share some common vision of resolution. This is clearly not present when domestic violence has taken place in a relationship.
Mediation practitioners must be alert to the need to interview partners separately with specially designed questions in order to determine if abuse is or has been present. Many domestic violence professionals can train others to screen safely for domestic violence. To not do so risks unsuccessful mediations, at best, and increasing the victim's danger by colluding with the abuser, at worst.
A person who has been terrorized by an abuser is not free to participate in a mediation process with him, even if the mediator(s) assume or believe that they "understand". Being truthful about any of her needs or experiences in the abuser's presence or proximity practically ensures that she is in more danger later.
The mediator is left with a no win: either the victim's danger is increased, or she is not fully or truthfully participating, or both. The well meaning mediator may actually encourage the victim to feel safe enough to share information that could seriously compromise her safety. In any case the whole intent of mediation is lost.
To engage an abuser and a victim in a process that implies equal responsibility is damaging to both. The victim is once again made to feel responsible for the abuser's behavior, and the abuser is allowed to continue to not accept full responsibility for his behaviour choices.
Joint counselling is never recommended where there is any type of abuse within the relationship. He will simply use that to manipulate the mediator (he has certainly done a right number on you and I do think he has made any underlying problems you may have that stem from your own past far, far worse. I think he targeted you actually).
Individual counselling solely for you is however, an excellent idea.
He should no longer come and visit you at your residence, he is basically marking his territory by doing that. You are also compounding that error here by further sleeping with him. That also needs to stop, you are really sleeping with the enemy.
WA and SSAFA are certainly worth contacting now and they can both help you get away from your abusive H. It will do your children no favours at all to be brought up seeing their dad abuse you as their mother; they could well go onto be abusers or abused themselves. That is certainly no legacy to leave your children.
Better to be alone as well than to be badly accompanied. You and he need to be apart before he completely destroys you and drags your children down with him.